The reduction in memory cell size required for high density dynamic random access memories (DRAMs) results in a corresponding decrease in the area available for the storage node of the memory cell capacitor. Yet, design and operational parameters determine the minimum charge required for reliable operation of the memory cell despite decreasing cell area. Several techniques have been developed to increase the total charge capacity of the cell capacitor without significantly affecting the cell area. These include structures utilizing trench and stacked capacitors, as well as the utilization of new capacitor dielectric materials having higher dielectric constants.
Capacitors comprise two electrically conductive plates separated by an intervening capacitor dielectric layer. An example theoretical ideal construction is shown in FIG. 1. There illustrated is a wafer fragment 10 having a mass or layer 12 of oxide, such as undoped SiO.sub.2 deposited by decomposition of tetraethylorthosilicate (TEOS). A patterned lower electrically conductive capacitor plate 14 overlies oxide layer 12. A cell dielectric layer 16 overlies lower plate 14, with an outer conductive plate. 14 being shown in the form of an electrically conductive layer 18. Common preferred materials for layer 16 are electrically insulative nitrides, specifically Si.sub.3 N.sub.4.
Advanced low pressure chemical vapor deposition techniques for providing silicon nitride unfortunately provide some deposition selectivity of the nitride for underlying silicon as opposed to underlying oxide. Conductively doped polycrystalline silicon is a typical and preferred material of construction for a capacitor storage node 14. The undesired selectivity of the deposition creates adverse thickness depletion of the deposited nitride dielectric layer, as shown in FIG. 2 at location 15. Like numerals from the first described embodiment are utilized where appropriate, with differences being indicated by the suffix "a". As is apparent, the thickness of deposited nitride layer 16a is significantly depleted at the interface or transition edge between the underlying oxide and silicon of storage node 14. This undesirably results in bread loafing of the outer capacitor plate layer 18a. Even more problematic, the very thin nature of the cell dielectric layer 16a at the oxide silicon interface 15 results in reduced capacitance at best and a catastrophic poly 18a to poly 14 short at worst.
One prior art solution to this problem is to provide underlying oxide layer 12 to be plated or coated with a layer of nitride, such as Si.sub.3 N.sub.4. However, integrating deposited nitride into a fully integrated circuit process flow poses difficulties.
FIG. 3 illustrates a more typical prior art construction in the context of a DRAM memory cell where the problem is found to manifest. FIG. 3 illustrates a wafer fragment 20 comprised of a bulk monocrystalline silicon substrate 22 and a pair of adjacent word lines 24. Word lines 24 are encapsulated in an oxide material 26 (SiO.sub.2) to provide electrical insulation thereof. A conductively doped diffusion region 28 is provided relative to monocrystalline silicon substrate 22 intermediate word lines 24. A capacitor construction 30 is provided intermediate word lines 24 and in ohmic electrical connection with diffusion region 28. Such comprises an inner or lower capacitor plate 32, typically formed of conductively doped polycrystalline silicon. A nitride cell dielectric layer 34 is provided outwardly of substrate 22, oxide regions 26 and lower capacitor plate 32. Again and as shown, such results in undesired thickness depletion of layer 34 at the transition edge or interface of storage capacitor plate 32 and encapsulating oxide 26. A cell polysilicon layer 36 is provided outwardly of cell dielectric layer 34. Again the thickness depletion results in reduced capacitance or a fatal short of the capacitor plate through the dielectric layer.
It would be desirable to overcome the above described prior art problems of thickness depletion involving nitride dielectric layers and silicon capacitor plates overlying oxide layers without having to resort to substitution of nitride for oxide.